This invention relates to a contactless seal and, particularly, to a contactless seal between a rotatable part and a stationary part.
As is known, various types of contactless seals have been used to seal a gap or space defined between a stationary part and a rotating part. For example, one such contactless seal has been known to use a sealing member which surrounds the axis of rotation of the rotatable part with at least two sealing surfaces concentric to the axis and which is movably disposed on the stationary part. The sealing member also has a pressure chamber which is open towardsthe rotatable part and is connected to a supply of a barrier medium. This pressure chamber is further formed in the sealing member between two adjacent sealing surfaces, and the sealing member is pressed by a pressure force against a surface on the rotatable part so as to leave gaps for the barrier medium to escape between the cooperating sealing surfaces.
A seal of this kind is disclosed in U.S. Pat. No. 3,606,568, which relates to sealing of a water turbine and which has sealing surfaces lying on a large radius (e.g. 2 to 5 meters), the speed of the rotating part relative to the stationary part being up to 150 meters per second. The barrier medium is filtered water, thus preventing sandy water, if present, from flowing out of the turbine housing. The barrier medium is supplied to the pressure chamber without throttling and the pressure of the barrier medium in the pressure chamber is constant. The gap width between the sealing surfaces should be adjusted to an equilibrium width since the force tending to widen the gap which is caused by the pressure of the barrier medium therein decreases as the flow speed increases, i.e., as the gap width increases. This is due to the Bernoulli law, which holds that the pressure of a medium decreases with the square of the increasing flow speed. Thus, when this seal is used, the gap width must be relatively large. However, this results in corresponding losses of barrier medium.
Accordingly, it is an object of the invention to provide a contactless seal which can operate in a state of equilibrium with very small gap widths.
Briefly, the invention provides a contactless seal for sealing a space between a stationary part and a rotatable part which is disposed to rotate about an axis of rotation. The contactless steel includes a sealing member which is disposed about the axis of rotation and has at least two sealing surfaces concentric to the axis of rotation and spaced from the rotatable part to define gaps therebetween for the escape of barrier medium. The sealing member is also movably mounted on the stationary member for movement under a biasing force towards the rotatable member. In addition, the seal includes a pressure chamber in the sealing member between the sealing surfaces, which chamber is open towards the rotatable part and is connectable to a supply of barrier medium. A plurality of transverse webs are located in the chamber to sub-divide the chamber into a plurality of individual sub-chambers distributed peripherally of the sealing memer. Each of these individual sub-chambers is connected to the supply of barrier medium to receive a flow of barrier medium therefrom. Also, a plurality of restrictors are provided with each restrictor being disposed between a respective one of the individual sub-chambers and the supply of barrier medium to throttle the flow of barrier medium to the respective individual sub-chamber.
As a result of the aforementioned seal structure, the prior-art pressure chamber, which was continuous in the peripheral directions, is divided into a number of small individual chambers and the pressure of the barrier medium in the individual chambers varies with the gap width as a result of the upstream restrictors. If the gap width in the seal becomes larger, more barrier medium escapes, so that the pressure in the individual chamber concerned falls as a result of the throttle effect. This reduces the forces tending to open the gap, whereas the forces tending to narrow the gap and caused by the pressure acting on the sealing member remain constant. As a result, the sealing member has a stable equilibrium position with a very narrow gap width. This mechanism, which rigidly determines the gap width, operates uniformly over the entire seal periphery, since the webs between the individual chambers prevent circumferential compensating flows occurring from one chamber to the next. As a result, the seal is uniformly adjusted over the entire periphery to the desired gap width, even if the sealing surface of the rotating part is not completely flat but somewhat corrugated. In order to ensure the adaptability of the seal to irregularities in the sealing surfaces on the rotating part, the sealing member must have some flexibility. However, this usually results from the large sealing radius, even if the sealing member is made of conventional metallurgical material. Thus, since the seal operates with narrow gaps than the previously known seal, the losses of barrier medium are correspondingly lower, so that the seal can be used more economically.
According to one advantageous embodiment, a chamber is provided in the stationary part adjacent the sealing member and is connected to the supply of barrier medium, so that the barrier medium also exerts pressure on the sealing member. This simplifies the structure of the seal, since there is no need for a separate supply of pressure medium.
According to another embodiment, the sealing member is made up of a number of straight portions which form a polygonal ring or array which is guided axially in a correspondingly-shaped annular groove in the stationary part. In this embodiment, elastic intermediate members can be disposed between the straight portions, thus providing a particularly resilient embodiment of the seal.